This invention relates to heating apparatus comprising a plastics body and a planar heating element, and is particularly directed to the method of attaching the heating element to the body. The invention relates particularly, but not exclusively, to water heating apparatus such as electric kettles.
Various liquid heating appliances have been developed incorporating planar heating elements, and different techniques have been employed for the attachment of the heating element to the body of the heating apparatus. A seal is conventionally provided between the body of the heating apparatus and the heating element, and the attachment methods attempt to ensure a uniform compression of the seal in order to provide reliable sealing.
GB 2,291,325 discloses a method of attaching a planar heating element to a vessel body, which relies upon deformation of the vessel body around the heating element, so as to clamp the heating element in position while compressing a seal. One problem with this approach is that the deformation of the vessel body, which is achieved by heating the body and subsequently applying a force to the material, incurs structural weakness in the material. In the case of a plastics vessel body, subsequent creepage may occur reducing the compression held on the seal. Furthermore, the plastics deformation stage may not be controlled with sufficient accuracy to provide a known uniform level of compression of the seal.
According to the invention there is provided a method of attaching a planar heating element to a plastics body to define a liquid heating vessel, an aperture being defined in the body for receiving the heating element, the method comprising the steps of:
covering the aperture with one face of the heating element and positioning a seal between the heating element and the body;
positioning a retaining ring over the other face of the heating element, and which engages with a portion of the plastics body, an induction heating member being located between the body and the retaining ring;
causing inductive heating of the member to form an induction weld of the ring to the body, while applying a force to cause relative movement of the retaining ring and the vessel body to clamp the heating element in position and to compress the seal.
The invention also provides a heating apparatus comprising a plastics body and a planar heating element together defining a liquid heating vessel, the heating element being clamped between a portion of the body and a retaining ring, the body and the retaining ring being fixed together by an induction weld.
In the method and apparatus of the invention, the heating element is clamped between the plastics body and a retaining ring, and these two components are held together by an induction weld. The use of induction heating enables the retaining ring and the body to be moved relatively to each other as the plastic softens, so that compression of the seal may be obtained. After cooling of the induction weld, the compression is maintained on the seal. The induction weld is less susceptible to creepage than a plastic swaging operation, and also provides a more uniform compression of the seal than the use of a number of discrete securing members, such as screws.
Preferably the body defines a collar extending around and below the aperture, and the retaining ring has a channel which is fitted over the collar, the induction heating member being located between a portion of the collar and the portion of the channel. This arrangement facilitates the location of the retaining ring. The body may be provided with stops around an outside face of the collar, and the retaining ring is then caused to move during induction welding until a portion of the retaining ring abuts against the stops, thereby defining a predetermined compression of the seal. This arrangement avoids the need for accurate process control in order to obtain the desired seal compression for effectively sealing the element in the aperture.
An intermediate shielding element may be provided between the heating element and the retaining ring. For an effective induction welding joint, the material of the two components being joined should be the same. Consequently, it may be desirable to shield the retaining ring from the heat generated by the heating element, and the intermediate element is provided for this purpose.
The seal between the body and the element may be provided around the outer periphery of the heating element before the heating element is positioned over the aperture.
The induction heating member preferably comprises a continuous metal ring disposed in the channel, and the inductive heating is caused by generating a radio frequency alternating magnetic field in the vicinity of the heating member. The use of a continuous ring allows induced currents to flow around the heating member, and this reduces the required frequency for the induction welding process, simplifying and reducing the cost of the equipment needed. The frequency is approximately 50 KHz.